Various techniques have been set forth for measuring the dielectric constant or electric permittivity of subsurface formations. Prior investigators have recognized that the relative dielectric constant of the different materials of earth formations vary widely (e.g. 2.2 for oil, 7.5 for limestone, and 80 for water) and that the measurement of dielectric properties therefore holds promise of being a useful means of formation evaluation. Since hydrocarbons and fresh water can have similar resistivity, the contrast between the dielectric constant of hydrocarbons and water is especially meaningful in situations where low salinity is encountered.
There has been recently developed a logging device which investigates earth formations surrounding a borehole by radiating microwave electromagnetic energy into the formations and then taking measurements which relate to the propagation of the energy in the formations. A form of this device, currently designated as an "electromagnetic propagation tool" (or "EPT") is disclosed in the U.S. Pat. No. 3,944,910. In operation of the EPT logging device, the relative phase of energy received at a spaced receiver pair is measured and used to obtain indications of the formation dielectric constant, typically in the so-called invaded zone of the formations which are nearest the borehole. (At a fixed frequency of operation, relative phase and travel time per unit distance are proportionally related, and travel time will be referred to in place of phase in this background discussion.) The wave attenuation may also be measured at the receivers and used to implement corrections to the EPT travel time measurements since the lossiness of the propagation medium can affect travel time therethrough.
Since the dielectric constant of water is much higher than that of hydrocarbons or formation matrix material, the travel time will be largely dependent upon the fraction of water in the formations; i.e., the water-filled porosity of the formations, designated .phi..sub.w. In the above-referenced U.S. Pat. No. 3,944,910, and in U.S. Pat. No. 4,092,583, there are disclosed techniques for obtaining .phi..sub.w from travel time (and attenuation, where applicable) measurements taken with an EPT logging device.
While the EPT has proved to be quite effective in obtaining measurements of properties of subsurface formations surrounding a borehole, viz. in the invaded zone thereof, it has been found that presence of a substantial mudcake can sometimes give rise to inaccuracies. The EPT has a relatively short spacing between transmitter and receivers and an attendant shallow depth of investigation. It therefore stands to reason that such a shallow investigation measurement device will necessarily be "looking" at mudcake to some extent. Techniques for determination of mudcake thickness and/or for correction of the effects of mudcake thickness have been set forth in the prior art in the context of certain logging devices, such as acoustical logging devices. For example, in the U.S. Pat. No. 3,608,373, there is disclosed an acoustic logging device which utilizes the difference between direct and differential pulsed acoustic travel times to obtain mudcake thickness. The use of a difference follows directly from a difference in travel paths of the acoustic wave; i.e., one path (the differential) having travel time through the mudcake cancel out, whereas the other path (direct) includes travel time through the mudcake. This simplified plane-wave type of model is not sufficient, however, when considering the effects of mudcake on an EPT device wherein continuous wave electromagnetic energy, rather than acoustic energy, is measured at the receivers in the relatively "near field" of the wave energy. The spacings of the EPT device necessitate a "near field" phenomenon analysis which takes into account electrical properties of both the mudcake and the formations that will affect the field measured at the receivers. In particular, both the conductivity and the dielectric constant of the mudcake and the formations, as well as the mudcake thickness, will affect the field detected at the receivers of the logging device. Accordingly, simple relationships between travel times will not yield a meaningful mudcake correction for the EPT.
It is one of the objects of the present invention to provide an apparatus and method for determining mudcake-corrected EPT-determined measurements of formations surrounding a borehole.